Abstract The uptake of l-tyrosine at various stages of development was examined in synaptosomes and in plasma membrane vesicles derived from rat bruin. The total uptake has two components. Na + dependent and Na +-independent, respectively. The Na +-dependent component of the transport system appears around the 5th postnatal day and increases with age. The affinity of the transport system for tyrosine does not vary substantially during development. The V max increases more than six-fold between day 15 and adulthood. Plasma membrane vesicles derived from T 3-treated rats accumulate more tyrosine than those obtained from the control animals. The results support the view that thyroid hormones during development promote the establishment of the systems implicated in neurotransmission in the developing nervous system.